Initial commit

Author: VlachJosef

.gitignore

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+target/

build.sbt

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+lazy val root = project
+  .in(file("."))
+  .settings(
+    name := "scala3-tuple-cast-exception",
+    scalaVersion := "3.0.0-RC2"
+  )

project/build.properties

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+sbt.version=1.4.9

project/plugins.sbt

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+addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % "0.5.4")

src/main/scala/Main.scala

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+import scala.deriving.*
+import scala.compiletime.{erasedValue, summonInline}
+
+case class Simple(a: String, b: Boolean) derives Printable
+case class SimpleT(a: (String, Boolean)) derives Printable
+
+@main def hello: Unit = {
+
+  summon[Printable[Simple]].print // Prints STRING BOOLEAN as expected
+
+  summon[Printable[SimpleT]].print // java.lang.ClassCastException: SimpleT$$anon$1 cannot be cast to scala.deriving.Mirror$Product
+
+}
+
+trait Printable[T]:
+   def print: Unit
+
+object Printable:
+
+   given Printable[String] with
+      def print: Unit = println("STRING")
+
+   given Printable[Boolean] with
+      def print: Unit = println("BOOLEAN")
+
+   def printProduct[T](p: Mirror.ProductOf[T], elems: => List[Printable[_]]): Printable[T] =
+      new Printable[T]:
+         def print: Unit =
+            elems.foreach(_.print)
+
+   inline given derived[T](using m: Mirror.Of[T]): Printable[T] =
+      val elemInstances = summonAllPrintable[m.MirroredElemTypes]
+      inline m match
+         case p: Mirror.ProductOf[T] => printProduct(p, elemInstances)
+
+end Printable
+
+inline def summonAllPrintable[T <: Tuple]: List[Printable[_]] =
+   inline erasedValue[T] match
+      case _: EmptyTuple => Nil
+      case _: (t *: ts) => summonInline[Printable[t]] :: summonAllPrintable[ts]